Motor grader drive systems have been marketed wherein speed sensors are utilized to monitor and control the speed ratio between the speed of the hydrostatically driven front wheels relative to rear wheel speeds wherein the front wheels are driven by a hydrostatic motor in closed loop connection with a variable displacement hydrostatic pump and wherein pump displacement is varied in response to the speed ratio to modulate the speed of the front wheel drive. In the primary operational mode, an electronic control is utilized to modulate the hydrostatic drive so that the front wheel drive train is driven at a speed slightly less than rear wheel speed. With little or no slippage of the rear wheels, an overrunning clutch permits the front wheels to turn at ground speed. However, if the rear wheel slippage is greater than the speed difference between the front drive train and the rear wheels, the overrunning clutches engage to turn the front wheels at the slower speed. Therefore front wheel slippage is always slightly less than rear wheel slippage when the unit is operating in the primary operational mode.
This system has a second mode of operation referred to as an "aggressive" mode wherein the electronic control is utilized to drive the front wheels at a slightly greater speed than the rear wheels. This aggressive mode of operation is used primarily to improve steering control when the grader is in lower speed drive ranges. Overrunning capability of the hydrostatic motors is used to reduce torque interferences between the front wheel drive and the main transmission drive. In this system the aggressive control should not be used where tractive conditions are good such as on dry hard surfaces since it will result in loss of performance and efficiency.